There are a number of accepted treatment options for patients diagnosed with localized RCC. These include radical nephrectomy (whether open, laparoscopic or robotic), partial nephrectomy (whether open, laparoscopic, or robotic), surgical or non-surgical ablation, and active surveillance. The most appropriate treatment strategy will depend on patient (host) and tumor characteristics. These details are discussed more fully in the “Malignant Renal Tumors” article in the UroToday Center of Excellence series.
Kidney cancer has been historically thought of as a “radio-resistant” tumor. This is based on in vitro studies2 as well as the fact that early trial of adjuvant and neoadjuvant radiotherapy in patients with RCC undergoing surgical resection failed to show benefit.3,4 As a result, traditionally fractionated radiotherapy has been historically limited to palliative intent for patients with RCC. However, hypofractionated, high-dose radiotherapy has proven successful in the local control of RCC metastasis to the brain and other bony and visceral sites (refs 6-15). Coinciding with these clinical data was the emergence of data demonstrating the efficacy of high dose per fraction radiotherapy in the treatment of RCC in a mouse model.5 This led to increasing interest in the use of stereotactic body radiotherapy (SBRT) in the treatment of localized RCC. SBRT is routinely used for the treatment of malignancies of other tissue types including lung, liver, spine, and prostate.6 Compared to other radiation techniques, SBRT utilizes a smaller number of higher dose fractions. This is believed to assist with overcoming the previously believed radioresistance of RCC. Further, compared to other ablative approaches, one of the advantages of SBRT is the ability to treat larger lesions.6
Given uncertainties about both the efficacy and toxicity of such an approach, initial investigation has focused on patients in whom extirpative surgery, the gold standard approach, is not feasible or safe.
There are currently both retrospective and prospective reports characterising outcomes for patients treated with SBRT for localized RCC. These studies include a variety of treatment approaches including single fraction treatment (often 26 Gy in 1 fraction) and multiple fraction regimes (including regimes ranging from 2 to 10 fractions and total doses ranging from 5 to 85 Gy). As may be expected from some different treatment approaches, there are differences in both efficacy and toxicity between studies.
Prospective cohort studiesA recent systematic review identified eight published prospective studies of SBRT in the treatment of patients with localized RCC.7 Apart from one study published in 2006, the remainder have been published in the last five years. The strength of conclusions that can be drawn from these data are limited by small sample sizes (4 to 40 patients with localized RCC per study) and limited follow-up (13 to 52 months, with most 2 years or less).7 In addition, as previously mentioned, there were significant differences in total dose delivered and radiotherapy prescription between studies.
In each case, the authors report on patients who were either deemed medically inoperable, at very high risk for surgery due to the risk of dialysis or who refused surgery. Some studies had specific, disease-related criteria (e.g. a single lesion, maximal tumor dimension less than 4 or 5 cm) whereas this was not specified in other manuscripts. Outcomes were variably reported with local control most often reported. Additionally, adverse events were variously, and non-systematically reported.
Local control rates varied, in large part in correlation to the duration of observation: from 87% local control rate at a median 37 months follow up to 100% at two years in one trial8. Notably, even in the publication from Siva and colleagues who reported 100% local control, 10% of patients experienced distant progression, an outcome that is more likely to contribute to morbidity and mortality than local recurrence.8 Longer-term outcomes remain to be assessed.
Likely due in part to differing radiotherapy prescriptions, toxicity rates varied significantly. Both Siva and colleagues and Svendman and colleagues reported grade 1-2 toxicity in more than 50% of patients, most notably characterized by chest wall pain, nausea, and fatigue.8,9 In addition to considerations regarding comorbidity, SBRT and other non-surgical approaches to renal masses are often considered in patients with poor renal function for whom nephron preservation is a top priority. Thus, post-procedural renal function is an important outcome and, again, results vary between reports. Kaplan and colleagues reported worsening of renal function in 2 of 12 patients undergoing SBRT for medically inoperable tumors less than 5cm.10 McBride et al., in a similar population of patients, found that 2 of 15 patients (13%) had late grade 3 renal dysfunction with a mean decrease in glomerular filtration rate of 18 mg/dL among the whole study population.11 Similarly, Ponsky and colleagues demonstrated an 11% rate of grade 3 renal dysfunction among 19 patients deemed poor surgical candidates who received SBRT.12 Finally, and perhaps more optimistically, Siva and colleagues found in their cohort of 21 patients that the average decrease in glomerular filtration rate was only 8.7 mL/min at one year following treatment.8. Taken together, evidence suggests that increased fractionation (as in 20 to 30 Gy in 10 fractions) was strongly correlated with renal atrophy.13
Taken together, these data suggest that, for patients who receive three radiation fractions, a minimum per fraction dose of 11 Gy should be administered as this was the minimum dose that, in prospective cohorts, no patient experienced local failure.7
Retrospective cohort studies
In addition to the aforementioned prospective cohort studies, there are a number of retrospective cohort studies examining the use of SBRT in primary RCC. These, for the most part, have the same limitations are the prospective studies including limited sample size, short follow-up and heterogeneity of radiotherapy prescription. While most of these reports demonstrated local control rates comparable to the prospective literature (93 – 100%), one study demonstrated significantly lower local control (65%) among patients who had a history of radical nephrectomy for RCC in the contra-lateral kidney14. Those patients received 60 to 85 Gy in 5 to 7 fractions using stereotactic gamma-ray irradiation.
Patient selectionSurgery remains the mainstay of curative-intent treatment for patients with localized RCC. Ablative approaches, including SBRT, may, therefore, be considered among patients for whom surgery is contraindicated or who refuse surgery. Recent guidelines have recommended emphasizing that SBRT remains an experimental option in RCC due to the relatively limited worldwide experience and lack of long-term data.15
Treatment recommendationsThe International Radiosurgery Oncology Consortium for Kidney performed a 65 item survey among eight institutions who performed SBRT for primary RCC.16 A number of important conclusions came out of this work and the resulting consensus statement. First, all included centers treat patients with solitary kidneys or pre-existing hypertension. Five of the eight institutions have size cut off criteria ranging from 5 to 8 cm in maximal tumor dimension. The total planning target volume expansion varied between institutions, ranging from 3 to 10 mm. While all centers used pretreatment image verification, seven of the eight utilized intrafractional monitoring of some sort. Radiation prescriptions varied from 1 to 12 fractions with a total dose of 25 to 80 Gy. However, the consensus statement recommends a total dose of 36 to 45 Gy for patients receiving 3 fraction regimes and 40 to 50 Gy for patients receiving 5 fraction regimes. Obviously, the size of the primary tumour and its proximity to critical and adjacent structures will influence the total dose and fractionation regime.
Ongoing surveillance follow-up for local tumor response and recurrence varied with some institutions relying on computed tomography (CT) alone while others used magnetic resonance imaging (MRI) or PET-CT. Typically, follow-up was performed every three to six months in the first two years and every three to twelve months in the subsequent three years.
One of the challenges in the post-treatment monitoring of these patients is the interpretation of radiographic studies and identification of imaging studies. Among 41 tumours treated with SBRT, the largest available study of imaging characteristics following treatment found that the linear growth rate regressed by an average of 0.37 cm per year after treatment but that there were no significant changed in enhancement when comparing imaging before and following treatment.17
Stereotactic body radiotherapy is an emerging treatment approach for patients with primary renal cell carcinoma. Compared to other ablative approaches (such as radiofrequency ablation or cryotherapy), it offers the opportunity to treat larger tumors and potentially those in closer proximity to critical structures. To date, surgical extirpation via partial or radical nephrectomy remains the gold standard and SBRT has primarily been investigated among patients who are either deemed medically inoperable or who refuse surgery. There are a number of ongoing studies assessing the role of SBRT that will increase the prospective global experience with this approach, however, none will provide comparative data with other treatment approaches. One particular area of interest in the potential for synergistic effects between SBRT and systemic therapy, particularly immunotherapy (ClinicalTrials.gov identifiers: NCT01896271, NCT02781506, NCT02306954, and NCT02334709).